1. Field of the Invention
The present invention relates to a ring oscillator and a voltage controlled oscillator (VCO) able to be used for example in a phase locked loop (PLL) for high speed optical communications.
2. Description of the Related Art
There is known in the related art a ring oscillator which does not use any vibration element such as a quartz oscillator, but is comprised of an odd number of inverters IV.sub.1 to IV.sub.n connected as a ring, as shown in FIG. 1.
In such a configuration, by inputting control signals CTL.sub.1 to CTL.sub.n to the inverters IV.sub.1 to IV.sub.n from the outside, the propagation delay times of the inverters IV.sub.1 to IV.sub.n can be controlled and the frequencies of the output signals of the inverters IV.sub.1 to IV.sub.n can be adjusted.
In such a ring oscillator of the related art, however, the Q value is low, it is difficult to realize a duty of 50 percent, and there is jitter. Namely, it is not possible to obtain stable oscillation.